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CHAPTER 14
NMR Spectroscopy
Important Terms
applied magnetic field
the externally applied magnetic field.
chemically equivalent
protons with the same connectivity relationship to the rest of the molecule.
protons
chemical shift
location of a signal occurring in an NMR spectrum. It is measured downfield from
a reference compound (most often TMS).
13
C
NMR
nuclear magnetic resonance from carbon (
13
C
) nuclei.
COSY spectrum
a 2-D NMR spectrum showing
1
H
-
1
H
correlations.
coupled protons
protons that split each other’s signals. Coupled protons have the same coupling constant.
coupling constant
the distance (in hertz) between two adjacent peaks of a split NMR signal.
DEPT
13
C
NMR spectrum
a group of four
13
C
NMR spectra that distinguish
CH
3
, CH
2
,
and CH groups.
diamagnetic anisotropy
the term used to describe the greater freedom of
p
electrons to move in response
to a magnetic field as a consequence of their greater polarizability compared with
s
electrons.
diamagnetic shielding
shielding by the local magnetic field that opposes the applied magnetic field.
diastereotopic hydrogens
two hydrogens bonded to the same carbon that will result in a pair of diastereo-
mers when each of them is replaced in turn by deuterium.
2-D NMR
two-dimensional nuclear magnetic resonance.
doublet
an NMR signal that is split into two peaks.
doublet of doublets
an NMR signal that is split into four peaks of approximately equal height. A dou-
blet of doublets is caused by splitting a signal into a doublet by one hydrogen and
into another doublet by another (nonequivalent) hydrogen.
downfield
farther to the left-hand side of the spectrum.
effective magnetic field
the magnetic field that a nucleus “senses” through the surrounding cloud of
electrons.
enantiotopic hydrogens
two hydrogens bonded to a carbon that is bonded to two other groups that are
nonidentical.
Fourier transform NMR
a technique in which all the nuclei are excited simultaneously by an rf pulse, their
(FT-NMR)
relaxation monitored, and the data mathematically converted to a frequency spectrum.